In conventional electrophotography systems, a photoreceptor is supported by a mechanical carrier such as a drum or a belt. First, the photoreceptor is erased by exposure to an erase lamp which "bleeds" away any residual charge remaining on the photoreceptor from previous operations. The photoreceptor then is charged to a generally uniform charge, positive or negative, by subjecting the photoreceptor to a suitable charging device such as a corona or a charge roll. The charge distribution on the photoreceptor is then altered by the image-wise application of radiation, e.g., a laser, to the surface of the photoreceptor creating a latent image corresponding to the image-wise application of radiation on the photoreceptor. Toner is attracted to the photoreceptor in a pattern consistent with the charge distribution of the photoreceptor. The toner is then typically transferred, either directly or through an intermediate medium, from the photoreceptor to a receptor material or medium being printed, e.g., paper or film.
Such an electrophotography process enables the production of high quality images on the receptor material, such as film or paper. Apparatus which may utilize electrophotography include conventional laser printers, photocopiers, proofers, etc.
Monochrome printers produce a hard copy output in one toner color only, typically black. If the laser printer is to be used to print a different color, the conventional black toner cartridge is removed and replaced with a toner cartridge containing toner of another color, e.g., red. However, the laser printer still prints only a single color.
On the other hand, color printers use three primary colors, typically cyan, magenta and yellow, and in addition, optionally, black. Several techniques have been developed over the years to adapt electrophotographic techniques to use multiple colors.
U.S. Pat. No. 4,728,983, Zwadlo et al, Single Beam Full Color Electrophotography, assigned to Minnesota Mining and Manufacturing Company, the assignee of the present invention, discloses a method of making high quality color prints by electrophotography. A single photoconductive drum is used together with means to erase, electrostatically charge, laser-scan expose and toner develop during a single rotation of the photoconductive drum. In successive rotations, different colored images corresponding to color separation images are assembled in register on the drum. This assembled color image is transferred to a receptor sheet in a final rotation of the drum. Because a separate pass, i.e., rotation, is required for each primary color plane, at least four passes (rotations) are needed to obtain the final four color image print. Separate passes for each of the primary color planes significantly restricts the speed which a multiple color electrophotographic printing process can achieve.
Zwadlo et al and other similar apparatus have difficulty printing a multicolored image and then transferring such image directly or indirectly to plain paper or a transparency. It is necessary, following the teaching of Zwadlo et al, to print the desired image on an intermediate transfer material, such as a release liner or transfer adhesive, instead of plain paper or transparency film. Thus, these apparatus are not suitable for use in the general office printing market which must print on commonly available "plain" paper used in commercial offices.
In the Zwadlo et al system, the obstacle to printing on plain paper with an electrographic system is severe. For a standard four color image, four individual color planes of the final image must be laid down in registration on a photoreceptor. Each color plane must adhere first to the photoreceptor itself and, then, to the preceding color plane. Further each color plane, except the last, must survive intact without significant image degradation as subsequent color planes are laid on top. After all four color planes are assembled, all four color planes must be transferred intact to plain paper which may be medium commonly used in an office environment such as copier paper of various grades, weights and smoothness to transparency film.
Another difficulty in printing directly to plain paper arises from the thinness of the toner layers contained in the final image to be printed. Each toner layer may be less than 4 micrometers which makes the layer very fragile and difficult to manipulate. The thickness of a four color stack, approximately 4 micrometers, relative to the RMS roughness of paper, approximately 6 micrometers, makes it very difficult to effect complete image transfer to plain paper.
Other typical processes which print on plain paper rely on electrostatic assist to effect image transfer from an organic photoconductor. Examples of these processes are described in U.S. Pat. No. 4,728,983, Zwadlo et al; and U.S. Pat. No. 5,061,583, Zwadlo et al; U.S. Pat. No. 5,085,967, Usui et al; U.S. Pat. No. 5,115,277, Camis; U.S. Pat. No. 5,276,492; and U.S. Pat. No. 5,300,990, Thompson.